1. Field of the Invention
This invention relates to fiber lasers and more specifically to a tunable single-mode fiber laser using a MEMS Fabry-Perot filter that provides higher optical output powers and enhanced mode selectivity and stability.
2. Description of the Related Art
Rare-earth doped optical waveguides such as fibers or planar waveguides are used in amplifiers and lasers for telecommunications because they provide high optical gain over a broad spectral range. In the simplest laser geometry, a gain medium is placed in a cavity defined by two reflectors. The cavity has periodically spaced longitudinal modes with a frequency spacing Δν given by:                               Δ          ⁢                                           ⁢          v                =                  c                      2            ⁢            nd                                              (        1        )            where n the refractive index of the gain medium, c the vacuum velocity of light, and d the length of the cavity. The laser only oscillates at a frequency (or frequencies) that coincides with one (or several) of these cavity modes. Which one and how many modes reach threshold depends on the details of the gain medium. In the ideal case of a purely homogeneously broadened system only the mode that is closest to the gain maximum will oscillate and saturate the optical gain, i.e. pin the gain to the value that is necessary to reach the lasing threshold for this one mode. Even though rare earth doped glasses at elevated temperature are often considered to be dominantly homogenously broadened, inhomogeneous broadening is an important factor in these materials and for closely spaced longitudinal cavity modes, many lasing modes will oscillate.
To achieve single-mode operation, the active cavity length can be reduced so that the mode spacing exceeds the gain bandwidth. Since this approach limits the cavity length to a few hundred micrometers, the output power of such a laser is very small and typically tens of microwatts. [K. Hsu, C. M. Miller, J. T. Kringlebotn, E. M. Taylor, J. Townsend, and D. N. Payne, “Single-mode tunable erbium:ytterbium fiber Fabry-Perot microlaser”, Optics Letters, 19, 886 (1994), K. Hsu, C. M. Miller, J. T. Kringlebotn, and D. N. Payne, “Continuous and discrete wavelength tuning in Er:Yb fiber Fabry-Perot lasers” Optics Letters 20, 377 (1995)] With the development of waveguide/fiber Bragg gratings, at least one of the broad band reflectors can be replaced with a compact wavelength selective fiber Bragg grating, which provides feedback over a spectral width that is much narrower than that of the gain medium. With a typical spectral bandwidth of these reflectors of about 0.1-0.2 nm, active cavities as long as a few centimeters with output power of several tens of milliwatts have been demonstrated [W. H. Loh, B. N. Samson, L. Dong, G. J. Cowle, and K. Hsu, “High Performance Single Frequency Fiber Grating-Based Erbium:Ytterbium-Codoped Fiber Lasers”, Journal of Lightwave Technology, Vol. 16, No. 1, p. 114 (1998), D. L. Veasey, D. S. Funk, N. A. Sanford, and J. S. Hayden, “Arrays of distributed-Bragg-reflector waveguide lasers at 1536 nm in Yb/Er codoped phosphate glass”, Applied Physics Letters, Vol. 74, No. 6, p. 789 (1999)]. Longer cavities could provide even higher power output but also lead to a large number of longitudinal cavity modes inside the selected wavelength band and therefore to multimode operation of the laser.
Limited wavelength tunability can be achieved by controlling the temperature or length of the fiber Bragg grating to shift the spectral position of the reflection peak. Owing to the very small temperature dependence of the glass, the thermal tuning range of these lasers is on the order of a few nanometers only. Strain or compression tuning of specially designed fiber Bragg grating can result in larger tuning ranges. However, this kind of tuning is typically done using piezoelectric transducers and the effects of creep and hysteresis limit the wavelength reproducibility and so far have prevented the practical implementation of these lasers. Distributed feedback lasers also demonstrated single frequency operation with a high degree of side mode suppression. Due to the fixed grating, wavelength tunability of these lasers is, however, problematic as well.
Coupled cavity lasers, in which an external cavity (active or passive) is coupled to an active laser cavity, have the potential of combining mode selectivity with the possibility of wavelength tuning. The external cavity acts as a periodic wavelength dependent mirror providing minimum cavity loss only for certain longitudinal modes of the active laser cavity. The performance of such lasers depends on the relative optical length of the cavities where the distinction can be made between long-long and long-short cavity assemblies. In the case of a long-short coupled cavity, one cavity is short enough so that its mode spacing is large compared to the spectral width of the gain profile. All but the mode that is inside the gain profile of the active medium is suppressed. To tune the wavelength of such a laser over a region comparable to the spectral width of the gain spectrum, the optical length of the short cavity has to be changed considerably. An example of such a short external cavity fiber laser is given in U.S. Pat. Nos. 6,137,812 and 5,425,039. By placing a fiber assembly into fiber ferrule alignment fixtures, the length of a short air gap can be changed by piezoelectric means. Long-long cavities, on the other hand, have the advantage that only small changes in the optical path of either cavity are needed to obtain a broad tuning range. This effect is called the Vernier effect. In addition to that, long-long cavities are able to provide larger output powers. The major drawback of known long-long devices has been mode stability.
Ring lasers also have the potential of combining mode selectivity with the possibility of wavelength tuning and high output power levels (see “Tunable Erbium-Doped Fiber Ring Laser Precisely Locked to the 50-GHz ITU Frequency Grid”, Todd Haber, Kevin Hsu, Calvin Miller, and Yufei Bao, IEEE Photonics Technology Letters, Vol. 12, No 11, November 2000). The operation of ring lasers requires the use of additional optical components such as isolators, polarization controllers, wavelength division multiplexing (WDM) filters, and polarizers. To account for the insertion losses of these components, ring lasers typically require an active fiber length of at least 10 meters, and an overall fiber length of 15-20 meters. Haber mentions “To enable continuous single-frequency tuning operation of such a laser across a typical EDFA spectral width of 50 nm, a tunable filter of wide tuning range (>50 nm), narrow bandwidth (<1 GHz), and low loss (<3 dB) would be desirable. Unfortunately, no practical commercial filter at present can meet such stringent requirements . . . ” Instead, Haber cascades a fiber Fabry-Perot interferometer (FFPI) with a fiber Fabry-Perot tunable filter (FFP-TF). The FFP-TF selects one of the transmission peaks of the FFPI while the FFPI selects a single longitudinal frequency mode of the ring laser. The length of the active fiber, supports high output power levels but makes mode-stability difficult. That is apparent from Haber, where, even though the laser presented was operating at a single frequency and one polarization only, mode-hop free operation was observed for only 21 min at a time.
What is needed for most telecom applications is a high-power tunable single-mode laser, more particularly, a tunable laser that can function at power levels of 20 to 50 mW and provide stable single-mode operation over the C-band (1530-1565 nm) with rapid wavelength scanning over that band.